The term ‘light-duty combustion engine’ used herein broadly includes all types of non-automotive combustion engines including, but not limited to, two- and four-stroke engines used with hand-held power tools, lawn and garden equipment, lawnmowers, weed trimmers, edgers, chain saws, snowblowers, personal watercraft, boats, snowmobiles, motorcycles, all-terrain-vehicles, etc. Most light-duty combustion engines have some type of engine speed limiting device that prevents the engine from experiencing excessive speeds that could otherwise cause unwanted damage or catastrophic failure.
For instance, it is known to provide a light-duty combustion engine with a device that restricts the engine speed by simply stopping the combustion-initiating spark from being delivered to the combustion chamber when the engine exceeds a predetermined maximum speed. Though this approach is usually effective, under certain operating conditions the engine may experience a situation where an unintentional ignition occurs even though the ignition system has not provided any spark. This situation is generally referred to as ‘auto ignition’ and can include circumstances such as ‘dieseling’, ‘backfiring’, ‘active radical combustion’, ‘glow plug effect’, etc.
Typically, auto ignition occurs when the engine is being run at high temperatures and/or compression ratios such that the amount of energy present in the combustion chamber is sufficient to ignite air/fuel charges without requiring a spark. For example, when an engine operating at high speeds is suddenly turned off or otherwise shut down, there could hot spots or a glow plug effect in the engine that are hot enough to ignite nearby air/fuel charges; this is particular true during a compression stroke where the pressure within the combustion chamber is at elevated levels.